This invention relates to an air suspension piston that is constructed in the shape of a tube and is made of aluminum.
Air suspension systems are particularly used in the automotive engineering sector, e.g., in the case of automobiles or truck cabins. The characteristic feature of air suspension systems consists of an air cushion element in which air subjected to pressure is being used for cushioning.
The piston is an important component of an air suspension system. An air bellow, made of rubber, rolls on the outer surface of the piston when the air suspension system is in action and when a cushioning process takes place. The piston positions or supports the air bellow so that there are no folds. Furthermore, the geometry of the piston, namely the selection of various diameters of the piston, has an influence on the spring characteristics of the air suspension system.
During assembly of an air cushion system, the air suspension piston is fitted into the system components and for this reason the air suspension piston has to be engineered to high precision with minimum tolerances. High precision is also required by the fact that the air suspension piston is continuously exposed to pressure variations as a result of the action of the suspension system, with additional axial forces which act in a dynamic way being superimposed on the pressure variations.
Usually tubes are used as air suspension pistons; with these tubes being either made of steel or aluminium.
Insofar as steel tubes are used, they are longitudinal-seam welded and drawn. While such steel tubes meet the requirements of air suspension pistons, in particular concerning their dynamic characteristics, they are however associated with the disadvantage that steel tubes are relatively heavy.
An effort is therefore made to use aluminium tubes since they are considerably lighter, typically saving approximately 30% weight compared to steel tubes. In automotive engineering there is a general trend toward saving weight in the case of dynamically loaded components, because then all the other components whose design is influenced by the vehicle mass, can be of lighter construction. Thus, there is an endeavor to use aluminium, in particular if at the same time this leads to a reduction in the unsprung masses, of which the air suspension piston is a part, and which are so important for driving comfort.
Known aluminium air suspension pistons are made from extruded semifinished products. Due to the general precision requirements of air suspension pistons for the reasons explained above, the known aluminium tubes have to be drawn. For air suspension pistons, stringent manufacturing tolerances are of prime importance due to sealing against air loss, hence the above-mentioned requirements for precision which relate to both the exterior diameter and the interior diameter as well as the wall thickness of the aluminium tube. Due to the tolerances in the range of a few hundredths of mm applying to the fitted components, conditional on sealing problems as well as acoustic problems in the case of non pressed-on air suspension pistons, exacting requirements apply to the used semifinished products.
Furthermore, the exterior surface must be very smooth, so as to prevent excessive wear of the air bellows which are continuously rolling off this surface.
Moreover, for reasons of further weight saving, there is also the demand for minimum wall thickness of the tube of an air suspension piston. However, production of respective extruded aluminium tubes of such minimal wall thickness is only possible using a subsequent tube drawing process which is associated with considerable production effort and expense.
Thus the desired use of aluminium in the production of air suspension pistons is still associated with severe disadvantages and due to the above-mentioned requirements with regard to precision, wall thickness and minimum tolerances, the known extruded and drawn aluminium tubes require a great deal of production effort and expense.
Apart from extruded aluminium tubes, longitudinal seam welded tubes made of aluminium alloys are known per se. The German published application DE 41 42 325 A1 describes a method for producing chassis members for vehicles, in particular for automobiles, said chassis members being made of aluminium alloys. This published application is based on the fact that in those cases where the conventional extruded aluminium tubes are being processed to become chassis components of motor vehicles, many forming steps are required, including fluid forming or internal high-pressure forming.
As is described in the above mentioned published application DE 41 42 325 A1, the known extruded aluminium tubes are associated with the disadvantage that fluid forming after previous other forming is not possible because the forming capacity of the material has been exhausted by previous forming. Nor is it possible to carry out intermediate annealing prior to the fluid forming because the material either hardens at low annealing temperatures or becomes very soft at higher annealing temperatures, thus losing its strength. There is no possibility of regaining any of this strength by further heat treatment.
Against the background of nevertheless finding a starting material for the production of chassis members made of aluminium alloys, with which materials the necessary forming steps can be carried out without any problems, the above mentioned published application DE 41 42 325 A1 proposes a method which uses longitudinal-seam welded tubes from a self-hardened or hardenable aluminium alloy as blanks.